The invention relates to a process for seam welding an overlapping sheet-metal seam with a predetermined welding pressure. The invention also relates to a resistance seam welding machine for carrying out the process.
It is known that when a sheet-metal seam is welded by the roller seam welding process the initial weld spot is often incompletely welded. This is particularly disadvantageous in the production of can bodies, as the affected body has to be removed from the production line.
The problem of the initially unwelded or poorly welded seam has a number of causes. First of all, the leading end of the can does not benefit from any transfer of heat from a length of seam which has already been welded, and second, the insertion of the can body between the two roller electrodes, which forces these electrodes apart, causes a force to be exerted on the seam which is additional to the welding force which has been set, owing to the inertia of the electrodes. Furthermore, excessive welding pressure is applied at the edge of the can body, as, with welding force constant, less surface area is acted on than along the seam.
The increased welding force and/or increased welding pressure at the start causes a marked reduction in the electrical resistance in the sheet between the roller electrodes. Since the welding current is constant, the power input to the weld seam at the start of the can body changes in proportion to the said reduction in resistance. The measure adopted hitherto to counteract this has been to increase the welding current at the commencement of welding. The power input P to the sheet-metal is equal to the square of the welding current multiplied by the sheet resistance. Hence the reduction in sheet resistance at the commencement of welding can be compensated by a corresponding boost in current. However, the number of can bodies welded per minute is continually being increased. The can bodies are being fed into the welding plane more and more rapidly, and ever-increasing peak forces are occurring which in many cases can no longer be satisfactorily compensated by increasing the current.
A further problem is mechanical vibration of the welding electrodes which is set off by the peak forces at the starting edge of a can body. Efforts have been made to keep this vibration as small as possible by reducing the mass of one of the electrodes.